Mithril Aggregator produces a multi signature

[jpraynaud]

The Mithril Aggregator must be able to receive single signatures from Mithril Signers and to produce a multi signature that is embedded in a certificate with the following assertions:

• The real Mithril cryptographic library is used
• Stake distribution is static and fake
• Signer keys are static

1. Receive and validate single signatures

• Compute the digest and verify a received single signature with a MultiSigner trait and its implementation (and config & DI)
• Use this MultiSigner in the register_signatures http handler
• Add the verified single signatures to the MultiSigner state

2. Produce a multi signatures and a certificate

• Wait for the quorum and then compute the multi signature with the MultiSigner
• Produce a certificate associated to the multi signature
• Use this MultiSigner store in the certificate_certificate_hash http handler

3. Will be done later

• (Maybe it is not the best option) Notify the Snapshotter of the newly produced certificate hash so that it triggers the production/upload of the snapshot archive (check if it is needed)